Solid state image sensors have replaced conventional film for capturing images in cameras. The image sensor typically includes a two-dimensional array of pixels. Each pixel includes a photodiode that records the light received at one point in the scene that is being recorded. To capture an image, each pixel is reset prior to the scene being imaged onto the sensor. After a predetermined exposure time, the charge stored by each photodiode is readout to provide an image of the scene.
In a conventional camera, the exposure is controlled by a shutter that is triggered in response to the user pushing a button. The image sensor is reset just before the shutter opens and is readout at a predetermined time after the shutter closes. These operations are synchronized to the user pushing the button.
In some applications, the imaging array must determine when the exposure begins and ends without the aid of a synchronization signal such as the user pushing a button. For example, there has been considerable interest in replacing x-ray film images used in dentistry with digital images generated by CMOS image sensors. In these systems, the film that is placed in the patient's mouth is replaced by a CMOS imaging array that is covered with a layer of scintillation material that converts the x-rays to visible light that can be detected by the image sensor. Conventional x-ray systems using film do not require that the film exposure be synchronized with the x-ray source, since the x-ray pulse determines the exposure. Hence, conventional x-ray systems lack a synchronization system that can be used by the solid state image sensor.
It should be noted that the film does not accumulate a significant background exposure either before or after the film is exposed to the x-rays, since the film is in a light-tight cover. CMOS sensors, in contrast, exhibit a significant “dark current” that results in a low level of charge being accumulated on the pixel in the dark. This background increases the exposure needed to provide a dental x-ray if the background is not eliminated. Accordingly, the image sensor must be reset as close to the beginning of the x-ray pulse as possible to minimize the dark current background that accumulates before the image is formed. In addition, it is advantageous to detect the end of the exposure and trigger the readout as close to the end of the x-ray as possible to minimize the dark current background that accumulates after the exposure.
A number of systems have been proposed to deal with the synchronization of the imaging sensor with the x-ray pulse. The most straightforward approach would be to provide a synchronization signal similar to the pushbutton on a conventional camera. The imaging array could then be reset and the x-ray source triggered in the proper time sequence to minimize the exposure to the patient. Unfortunately, this strategy requires that the existing millions of x-ray machines already in place in dental facilities be modified at a considerable cost. Hence, some other form of triggering system has been sought.
In one class of triggering system, a separate set of detectors is used to detect the beginning of the x-ray exposure and trigger the reset, image acquisition, and readout when x-rays are detected. These additional detectors typically include additional photodiodes that are placed around the image sensor and are monitored to determine the start of the exposure. This type of system has three problems. First, the area of the separate sensors is relatively small, and hence, the sensitivity of the detection is less than ideal. In essence, the exposure sensors are equivalent to a few extra pixels in the image plane. The position of these sensors is behind the teeth or jaw bone, and hence, these sensors accumulate charge at a rate similar to that of the image during the image exposure. Hence, the detection time for the start of the exposure can be a significant fraction of the image exposure time. The x-ray exposure during this detection time is wasted, and hence, the x-ray dose to which the patient is exposed is longer than necessary.
Second, the sensors do not sample the entire image, and hence, the triggering decision is made on data that is not necessarily representative of the image. Third, to detect the end of the exposure, these detectors must be continually read out without interfering with the accumulation of the image by the sensors in the imaging array. Hence, the sensors are often separate from the array. Providing a separate set of sensors with each imaging array increases the cost of the imaging system.
In another class of prior art system, the imaging array is continually cycled. During each cycle, the imaging array is reset, allowed to accumulate charge for a predetermined period of time and then readout. If the image that is readout indicates the accumulation of a significant charge above that expected from the dark current, the system assumes that the exposure has begun, and the array is reset and allowed to accumulate the final image. The end of the exposure is set by a timer, rather than by detecting the end of the x-ray pulse in this type of system. Hence, there is some additional dark current accumulation after the x-ray pulse is turned off. This system has a better signal-to-noise ratio than systems based on a few small sensors, since the charge from a more representative set of photodiodes in the actual image is added together to make the triggering decision. Unfortunately, this system has high power consumption due to the repeated readout cycles. The high power consumption is particularly problematic in applications that rely on battery power. In addition, the detection time is increased by the time needed to readout each image during the detection phase. Finally, this system does not detect the end of the x-ray exposure.